If he is a tax resident of Thailand, he is in 99% of all cases not a tax resident of his home country. Double tax treaties are completely irrelevant. He only pays his taxes *here*.
But that has nothing to do with the term "tax resident". Especially as he is in most cases no longer a tax resident in his home country, when he is one in Thailand. There are only two countries as exceptions: USA and Ethopia. And one kind of silly exception, but worth noting: in many countries the amount of days to become a tax resident is 185. So technically one could for example be a ta resident in Thailand (180 days) *AND* Germany (185 days).
Learn the prices of goods and services, just because it looks like half the price as at home, it might still be a factor 5 rip off. And keep in mind: Thailand has mostly a "no tips culture". The only wide exception is Thai massage.
Get the Bolt app, it is a car/ride sharing app. It is most of the time cheaper than Taxi, and the cars are usually new electric cars. Thai/English translation is build in, and most drivers speak at least a bit English (most taxi drivers don't).
Someone mentioned the True Money App, but most "banking apps" will do and allow to connect a foreign credit card. Except for tourist shops or super big super markets, hotels etc. you will have bad luck paying with a debit or credit card. You have to withdraw cash, or pay with a banking app via QR code scan.
Anonymous participant 3 sentences what the company is doing, three sentences what you are doing for the company, and 3 or 4 copies of bank statements of received payments.
If in doubt, then ask the embassy/consulate and not on FB. They ask you to provide papers: they know what papers they want.
You probably did not point out clearly that you are employed? And they have the impression you are a free lancer/self employed ...
Work portfolios are usually asked from freelancers, not from employed workers!